Spatial Light Modulators for Phased-Array Applications

ABSTRACT

A capacitive micro-electromechanical system (MEMS) structure or device and methods of making and operating the same are described. Generally, the MEMS device provides a large stroke while maintaining good damping, enabling fast beam steering and large scan angles. In one embodiment, the capacitive MEMS device includes a bottom electrode formed over a substrate; an electrically permeable damping structure formed over the bottom electrode, the electrically permeable damping structure including a first air-gap and a dielectric layer suspended above and separated from the bottom electrode by the first air-gap; and a plurality of movable members suspended above the damping structure and separated therefrom by a second air-gap, each of the plurality of movable members including a top electrode and being configured to deflect towards the bottom electrode by electrostatic force. Other embodiments are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/008,772, filed Jun. 14, 2018, which claims priority under 35 U.S.C.119(e) to U.S. Provisional Patent Application Ser. No. 62/551,703 filedAug. 29, 2017, which is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present invention relates generally to spatial light modulators(SLMs), and more particularly to SLMs and methods for operating the samein phased-array applications.

BACKGROUND

Optical phased-array are widely used in a number of differentapplications including Light Detection and Ranging (LIDAR) systems inwhich beams of electromagnetic radiation or light are steered and lightreflected from objects scanned to detect and determine the range ofobjects detected by the LIDAR system. Typically, optical phased-arraysused in LIDAR systems require fast beam steering and large scan angles.

One type of spatial light modulators (SLM) used in optical phased-arraysin LIDAR systems is a mechanical SLM, such as a laser scanner thatincludes a spinning or moving mirror to steer the light beam.Unfortunately, these mechanical SLMs are rather bulky and the relativelylarge mass of the mirror limits the speed with which the light beams canbe steered or scanned.

Another type of SLM suitable for use in optical phased-arrays is adigital micromirror device (DMD) based SLM in which several hundred ifnot thousands of microscopic mirrors arranged in an array areelectrostatically pivoted or tilted in response to electronic signals.Although capable of providing much faster beam steering than themechanical SLM, achieving large scan angles requires small DMDdimensions, approaching the wavelength of the radiation being scanned.This in turn makes it difficult to maintain the speed advantage of theDMD-based LIDAR system.

Accordingly, there is a need for a SLMs and a method for operating thesame to provide fast beam steering and large scan angles forphased-array applications.

SUMMARY

In a first aspect a capacitive microelectromechanical system (MEMS)device, such as a ribbon-type spatial light modulator is provided havinga large stroke while maintaining good damping, thereby enabling fastbeam steering and large scan angles. In one embodiment, the MEMS deviceincludes a bottom electrode formed over a substrate; an electricallypermeable damping structure formed over the bottom electrode, theelectrically permeable damping structure including a first air-gap and adielectric layer suspended above and separated from the bottom electrodeby the first air-gap; and a plurality of movable members suspended abovethe damping structure and separated therefrom by a second air-gap, eachof the plurality of movable members including a top electrode and beingconfigured to deflect towards the bottom electrode by electrostaticforce.

In another aspect, an efficient method for driving the capacitive MEMSdevice is provided. Generally, the method involves ganging together alarge group of MEMS pixels, each comprising a plurality ofelectrostatically deflectable narrow ribbons, and driving each group ofMEMS pixels with a repetitive pattern to steer a beam of light reflectedfrom the ribbons.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be understood more fully fromthe detailed description that follows and from the accompanying drawingsand the appended claims provided below, where:

FIGS. 1A and 1B illustrate schematic block diagrams of an embodiment ofa ribbon-type SLM;

FIG. 2 is a schematic representation pitch and amplitude of deflectionof ribbons in a ribbon-type SLM to steer light reflecting from a surfaceof the ribbon-type SLM;

FIG. 3 is a graph of intensity versus steering angle and illustrates thesuitability of the ribbon-type SLM represented in FIG. 2 for phasedarray applications;

FIG. 4 is a schematic diagram modeling a ribbon of ribbon-type SLM as acapacitor-on-a-spring;

FIG. 5 is a block diagram illustrating a portion of a ribbon-type SLMduring fabrication and including damping structure with an electricallypermeable structure and a second air-gap under the ribbon;

FIG. 6 is a flowchart illustrating a method of fabricating a ribbon-typeincluding damping structure with an electrically permeable structure anda second air-gap under the ribbon;

FIG. 7 is a block diagram illustrating a ribbon-type SLM with a dampingstructure fabricated according to the embodiments of FIGS. 5 and 6;

FIG. 8 illustrates a simulated response of the ribbon-type SLM of FIG.7;

FIG. 9 illustrates a simulated response of a ribbon-type SLM without adamping structure;

FIG. 10 illustrates a simulated improved dynamic response of theribbon-type SLM of FIG. 7 with the damping structure at 100 KHz;

FIGS. 11A and 11B illustrates how increasing the period of pixels in aribbon-type SLM used in phased-array beam scanning increases angularsteering; and

FIG. 12 is a schematic block diagram illustrating architecture of aribbon array, drivers and driver bus of a ribbon-type SLM.

DETAILED DESCRIPTION

Embodiments of microelectromechanical system (MEMS) based spatial lightmodulators (SLMs), and methods for operating the same for use inphased-array applications, such as Light Detection and Ranging (LIDAR)systems, is disclosed. In the following description, numerous specificdetails are set forth, such as specific materials, dimensions andprocesses parameters etc. to provide a thorough understanding of thepresent invention. However, particular embodiments may be practicedwithout one or more of these specific details, or in combination withother known methods, materials, and apparatuses. In other instances,well-known semiconductor design and fabrication techniques have not beendescribed in particular detail to avoid unnecessarily obscuring thepresent invention. Reference throughout this specification to “anembodiment” means that a particular feature, structure, material, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. Thus, the appearances ofthe phrase “in an embodiment” in various places throughout thisspecification are not necessarily referring to the same embodiment ofthe invention. Furthermore, the particular features, structures,materials, or characteristics may be combined in any suitable manner inone or more embodiments.

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one layer with respect to other layers. As such,for example, one layer deposited or disposed over or under another layermay be directly in contact with the other layer or may have one or moreintervening layers. Moreover, one layer deposited or disposed betweenlayers may be directly in contact with the layers or may have one ormore intervening layers. In contrast, a first layer “on” a second layeris in contact with that second layer. Additionally, the relativeposition of one layer with respect to other layers is provided assumingoperations deposit, modify and remove films relative to a startingsubstrate without consideration of the absolute orientation of thesubstrate.

One type of MEMS based SLM suitable for use in a phased array of a LIDARsystem to modulate or steer a beam of light is a ribbon-type SLM, suchas a Grating Light Valve (GLV™), commercially available from SiliconLight Machines, in Sunnyvale Calif.

An embodiment of a ribbon-type SLM will now be described with referenceto FIGS. 1A and 1B. For purposes of clarity, many of the details of MEMSin general and MEMS optical modulators in particular that are widelyknown and are not relevant to the present invention have been omittedfrom the following description. The drawings described are onlyschematic and are non-limiting. In the drawings, the size of some of theelements may be exaggerated and not drawn to scale for illustrativepurposes. The dimensions and the relative dimensions may not correspondto actual reductions to practice of the invention.

Referring to FIGS. 1A and 1B in the embodiment shown the SLM 100includes a linear array 102 composed of thousands of free-standing,addressable electrostatically actuated ribbons 104, each having a lightreflective surface 106 supported over a surface of a substrate 108. Eachof the ribbons 104 includes an electrode 110 and is deflectable througha gap or cavity 112 toward the substrate 108 by electrostatic forcesgenerated when a voltage is applied between the electrode in the ribbonand a base electrode 114 formed in or on the substrate. The ribbonelectrodes 110 are driven by a drive channel 116 in a driver 118, whichmay be integrally formed on the same substrate 108 with the array 102.

A schematic sectional side view of a movable structure or ribbon 104 ofthe SLM 100 of FIG. 1A is shown in FIG. 1B. Referring to FIG. 1B, theribbon 104 includes an elastic mechanical layer 120 to support theribbon above a surface 122 of the substrate 108, a conducting layer orelectrode 110 and a reflective layer 124 including the reflectivesurface 106 overlying the mechanical layer and conducting layer.

Generally, the mechanical layer 120 comprises a taut silicon-nitridefilm (SiNx), and is flexibly supported above the surface 122 of thesubstrate 108 by a number of posts or structures, typically also made ofSiNx, at both ends of the ribbon 104. The conducting layer or electrode110 can be formed over and in direct physical contact with themechanical layer 120, as shown, or underneath the mechanical layer. Theconducting layer or ribbon electrode 110 can include any suitableconducting or semiconducting material compatible with standard MEMSfabrication technologies. For example, the conducting layer 110 caninclude a doped polycrystalline silicon (poly) layer, or a metal layer.Alternatively, if the reflective layer 124 is metallic it may also serveas the electrode 110.

The separate, discrete reflecting layer 124, where included, can includeany suitable metallic, dielectric or semiconducting material compatiblewith standard MEMS fabrication technologies, and capable of beingpatterned using standard lithographic techniques to form the reflectivesurface 106.

In the embodiment shown, a number of ribbons are grouped together toform a large number of MEMS pixels 126, each driven by a much smallernumber of drive channels 116.

FIG. 2 is a schematic representation of a portion of an array 202 in aribbon-type SLM shown in cross-section to long axes of the ribbons, andshowing a variation in pitch and amplitude of deflection of the ribbonsto steer light reflecting from a surface of the ribbon-type SLM.Referring to FIG. 2 it has been found that by ganging together a largegroup of MEMS pixels 204, each including multiple ribbons 206, anddriving each ribbon in a MEMS pixel to monotonically vary deflection ofeach ribbon from one end of the pixel to the next, it is possible toproduce a repetitive pattern across an array of pixel that can be usedto steer a beam of light. By monotonically varying deflection it ismeant that a first ribbon as one end of the pixel is deflected by afirst amount, while a second ribbon adjacent to the first is varied by asecond amount greater than the first amount, and a third ribbon adjacentto the second ribbon on a side opposite the first ribbon is deflected bya third amount greater than the second amount, and so on across theentire pixel. Furthermore, by using a SLM having a programmable MEMSpixel 204 pitch or spacing, by changing the number of ribbons 206 withina pixel it is possible to form and to continuously alter an angle oflight reflected from the SLM, such as is particularly useful in steeringa beam of light in phased-array applications, such as LIDAR.

FIG. 3 is a graph of intensity versus steering angle and illustrates thesuitability of the ribbon-type SLM represented schematically in FIG. 2for phased array applications. Referring to FIG. 3, it is seen that theperiodic spatial pattern along ribbon-type SLM shown in FIG. 2 creates aphased-array reflection, while varying the spatial period and amplitudeof the pattern changes the reflected beam angle, allowing theribbon-type SLM to rapidly cycle through patterns to sweep beam acrossfield. In particular, it is noted that as the period of the spatialpattern on the array 204 increases, i.e., as each period includes agreater number of ribbons, a maximum intensity 308 with which light isreflected from the array 202 shifts to the left as indicated by arrow310. As spatial period decreases or the number of ribbons in each periodreduced, the maximum intensity 308 with which light is reflected fromthe array 202 shifts to the right as indicated by arrow 312.

The high switching speed of the ribbon-type SLM makes it attractive forphased-array applications such as LIDAR. However, designing ribbon-typeSLM for LIDAR presents two challenges. First, a large stroke, i.e., anamount by which an individual ribbon can be deflected, is generallyrequired. Often it is desirable that the ribbon-type SLM have a strokeup to or exceeding the wavelength of the light being modulated orsteered. For example, it has been found a stroke of greater than 1 μm isdesirable to achieve adequate phase shift in applications for LIDARusing 1550 nm wavelengths. Thus, it is desirable that a ribbon-type SLMdesigned for LIDAR include an air gap under the ribbons that canaccommodate strokes of up to 2 μm.

A second challenge for ribbon-type SLMs used for phased-arrayapplications, such as LIDAR, is that the ribbons should include narrowribbon widths to achieve wide angular swing. Generally, it is desirablethat a ribbon used in ribbon-type SLMs for phased-array applicationshave a ribbon width of about 2.5 μm or less, and in some embodiments canbe as narrow as 0.5 μm.

These requirements of a large stroke and a narrow ribbon width make itvery difficult to switch the ribbon-type SLM at a high rate of speed,which is desirable for beam steering, because narrow ribbons over largeair gaps are very poorly damped and can behave like a guitar string,taking a long time to settle and thereby limiting the rate of speed atwhich the beam can be steered.

The impact of air gap and ribbon width on settling time will now bedescribed with reference to FIG. 4. FIG. 4 is a schematic diagrammodeling a ribbon of ribbon-type SLM as a capacitor-on-a-spring.Referring to FIG. 4, a voltage potential V(t) applied between a ribbon402 and a grounded lower or substrate electrode 404 creates anelectrostatic Coulomb attraction that deflects the ribbon a distance xtowards the substrate electrode. The electrostatic force is balanced byan elastic restoring force (represented by a spring 406 in FIG. 4). Theelastic restoring force, which is due to the taut silicon-nitride filmthe mechanical layer (shown as mechanical layer 120 in FIG. 1B), allowsthe ribbon 402 to revert back to a neutral state or position once theelectrostatic force is removed. In addition there is a damping force(represented by a damper 408 in FIG. 4), arising from squeeze-filmdamping which slows or damps movement of the ribbon 402 by the Coulombforce and the elastic restoring force. Squeeze-film damping is a strongfunction of both ribbon width and air-gap thickness. Settling time isproportional to the cube of the air-gap thickness, and inverselyproportional to the cube of the ribbon width. Thus, to achieve adequatedamping with narrow ribbons, it is desirable to have a very thinair-gap.

The Coulomb attraction force (F_(coulomb)) is given by:

$F_{Coulomb} = {\frac{1}{2}\frac{ɛ_{0}{AV}^{2}}{\left( {G - x} \right)^{2}}}$

where ε₀ is the permittivity of free space, A is the effectivecapacitive area of the ribbon in square meters (m²), G is gap thickness,and x is the linear displacement of the ribbon in meters, relative tothe substrate electrode.

The Elastic Restoring Force (F_(Elastic)) is given by:

F _(Elastic) =−kx

where k is the spring constant of the mechanical layer, and x is thelinear displacement of the ribbon 402, in meters, relative to thesubstrate electrode 404.

The Damping force (F_(Damping)) is given by:

$F_{Damping} = {{- b}\frac{dx}{dt}}$

where b is the damping constant of the air gap, and x is the lineardisplacement of the ribbon 402, in meters, relative to the substrateelectrode 404.

Thus, at equilibrium these three forces, Coulomb attraction, Elasticrestoring force and the Damping force, must balance.

However, as the ribbon 402 displaces past ⅓ a total thickness of the gap(G) between the ribbon in the neutral state and substrate electrode 404,the electrostatic force can overwhelm the elastic restoring force. Thisresults in a potentially destructive phenomenon commonly referred to as“snap-down” or“pull-in,” in which the ribbon 402 snaps into contact withthe substrate electrode 404 and sticks there even when the electrostaticforce is removed. Generally, it has been observed that snap-down occursat a characteristic displacement of x=G/3, where the ribbon 402 has beendeflected by one third of the original gap thickness. Thus, the ribbonsin conventional ribbon-type SLM are typically operated or driven to notbe deflected by a distance more than G/3 to prevent snap-down.Unfortunately, this leaves the lower ⅔ of the gap G empty, which in turnleads to poor squeeze-film damping.

Thus, to achieve adequate damping with narrow ribbons, it is desirableto create a very thin squeeze film gap, approaching the physical stroke(x) desired for the application, while to avoid pull-in it is desirableto create a much larger “electrical gap.”

Reducing the squeeze film gap while maintaining or increasing theelectrical gap can be done by inserting a dielectric between the ribbonand the substrate electrode. In one embodiment, a solid dielectric filmunderneath the ribbon is used to improve damping (and heat transfer) inthis way. For a dielectric thickness of G, the equivalent electricalthickness is G/ε_(r), where εr is the relative dielectric constant. Forexample, for silicon dioxide solid dielectric film having a relativedielectric constant of ε_(r)=3.9, and a vacuum or air gap having arelative dielectric constant of ε_(r)=1, to increase the electrical gapby 1 μm, it is necessary to provide nearly an additional 4 μm of adielectric material over the substrate electrode between the ribbon andsubstrate electrode. It is noted that integrating thick films, i.e.,films having a thickness greater than about 2 μm, into an existing MEMSprocess used to fabricate ribbon-type SLMs can be difficult orimpractical, since intrinsic film stresses can cause such thick films tovoid or delaminate, and film roughness can become excessive withincreased thickness. For this reason, a low dielectric constant materialis preferred.

In another embodiment, the squeeze film gap is reduced while maintainingor increasing the electrical gap by use of an electrically permeabledamping structure formed over the substrate electrode duringfabrication. Generally, the electrically permeable damping structureincludes a dielectric layer suspended above and separated from thesubstrate electrode by a first gap or first air-gap, where thedielectric layer defines at least a top surface of the air-gap. It isnoted that although this first gap is referred to as an air-gapthroughout the remainder of this disclosure, it need not be filled withair, but can alternatively be evacuated or filled with a mixture ofother gases without departing from the scope of the present invention.In some embodiments, the dielectric layer can substantially surround theair-gap to form a hermetic or hermetically sealed cavity. In otherembodiments, the first air gap is open to the MEMs environment,including a second gap or air-gap between the electrically permeabledamping structure and a lower surface of the ribbons, and the entireenvironment of the ribbon-type SLM can be evacuated or filled with fillgases and hermetically sealed. Suitable fill gases can include pure formor mixtures of one or more of Nitrogen, Hydrogen, Helium, Argon, Kryptonor Xenon.

A ribbon-type SLM including an electrically permeable damping structureand method of forming the same will now be described with respect toFIGS. 5 through 7. FIG. 5 is a block diagram illustrating a portion of aribbon-type SLM 500 with a damping structure including electricallypermeable structure and a second air-gap under the ribbon at anintermediate point during fabrication of the ribbon-type SLM. FIG. 6 isa flowchart illustrating a method of fabricating the ribbon-type SLMwith a damping structure. FIG. 7 is a block diagram illustrating aribbon-type SLM with a damping structure fabricated according to theembodiments of FIGS. 5 and 6.

Referring to FIGS. 5 and 6, the method begins with forming a bottom orlower electrode 502 over a wafer or substrate 504 (step 601). Generally,the substrate can include any suitable semiconductor or dielectricmaterial such as silicon, and the lower electrode can include one ormore layers of any suitable conducting material such as aluminum,copper, tungsten, titanium or alloys thereof and can be deposited usingany suitable CVD or PVD technique and patterned using standardphotolithographic techniques and etches. Optionally, as in theembodiment shown, the method can further include depositing a thinintermediate dielectric layer 506, such as a silicon oxide over thesubstrate 504 prior to forming the lower electrode 502.

Next, a layer of a sacrificial material is conformably deposited overthe substrate electrode 502 and patterned to form a first sacrificiallayer 508 (step 603). Generally, the sacrificial material of the firstsacrificial layer 508 can include any suitable material exhibiting aetch selectivity to the materials of the SLM and can be patterned usingstandard photolithographic techniques and etches. In one embodiment thesacrificial material of the first sacrificial layer 508 can include anamorphous silicon or polysilicon deposited by CVD to a suitablethickness. It is noted that the thickness of the first sacrificial layer508 determines a thickness of a first air gap (not shown in this figure)of the electrically permeable damping structure. Generally, this firstair gap is about ⅔ of an electrical gap between the ribbons 510 and thelower electrode 502. Furthermore, since a second air gap, which issubsequently formed between the ribbons 510 and the electricallypermeable damping structure is selected to have a thickness about equalto the maximum desired stroke; the thickness of the first sacrificiallayer 508 in one embodiment is about equal to twice the desired stroke.Generally the first sacrificial layer 508 has a thickness from about 0.2μm to about 2 μm.

Next, a dielectric material is deposited and patterned to form adielectric layer 512 of the electrically permeable damping structureover the first sacrificial layer 508 (step 605). This dielectric layer512 can include one or more layers of dielectric material such assilicon oxide, silicon nitride or silicon oxynitride and can bedeposited by CVD, atomic layer deposition (ALD) or, in the case ofsilicon oxides, can be thermally grown. The dielectric material ispatterned using standard photolithographic techniques and etches to formopenings through which the first sacrificial material is exposed forsubsequent removal. Generally the thin dielectric layer 512 has athickness from about 0.1 μm to about 0.5 μm.

Next, a conformal second sacrificial layer 514 is formed over thedielectric layer 512 (step 607).). As with the first sacrificial layer508, the sacrificial material of the second sacrificial layer 514 caninclude any suitable material exhibiting a etch selectivity to thematerials of the SLM and can be patterned using standardphotolithographic techniques and etches. In a preferred embodiment, thesacrificial material of the second sacrificial layer 514 is the same asthat of the first sacrificial layer 508 to enable it to be removed insingle etch step, after the ribbons 510 are formed. Thus, in oneembodiment the sacrificial material can include polysilicon deposited byCVD to a suitable thickness. As noted above, the thickness of the secondsacrificial layer 514 determines the thickness of a second air gapbetween the ribbons 510 and the electrically permeable dampingstructure, and is selected to have a thickness about equal to themaximum desired stroke. Generally the second sacrificial layer 514 has athickness from about 0.1 μm to about 1.0 μm.

Next, a plurality of ribbons 510 are formed over the second sacrificiallayer 514 (step 609). Generally, this involves two to three separatedepositions, beginning with deposition of a taut silicon nitridemechanical layer 120, a top or ribbon electrode layer 110, anddeposition of a reflective layer 124, as shown in FIG. 1B. The tautsilicon nitride mechanical layer can be deposited by CVD or ALD. The topor ribbon electrode layer can include any suitable conducting materialsused for the substrate electrode and can be deposited by PVD, CVD orALD. The reflective layer can include any suitable metal, dielectric orsemiconducting material capable of providing a light reflective surfaceat the desired frequencies and can be deposited by PVD, CVD or ALD,depending on the material. In some embodiments, the ribbon electrodelayer can include a metal that provides a light reflective surface toenable it to also serve as the reflecting layer. After deposition of themechanical layer, a ribbon electrode layer, and reflective layer, adrive bus 516 is formed, electrically coupling each ribbon electrode toa drive channel in a driver (not shown), integrally formed on the samesubstrate 502 with the ribbon-type SLM 500. Next, the mechanical,electrode and reflective layers are patterned or rib-cut using standardphotolithographic techniques and one or more etch steps to form theplurality of ribbons 510. It is noted that this patterning step exposesthe first and second sacrificial layers 508, 514 between the ribbons 510facilitating subsequent removal.

Finally, the first and second sacrificial layers 508, 514 are etched orremoved using a noble gas fluoride, such Xenon difluoride (XeF₂) to forma first air-gap (first air-gap 716 in FIG. 7) between a dielectric layer712 and a substrate electrode 702, and a second air-gap (second air-gap718 in FIG. 7) between the dielectric layer and a plurality of ribbons710, such that each of the plurality of ribbons are free to deflecttowards the substrate electrode upon application of a voltage potentialbetween the substrate electrode and the ribbon.

FIG. 8 illustrates a response of the ribbon-type SLM of FIG. 7 with theelectrically permeable damping structure. Referring to FIG. 8, line 802shows the drive voltage applied to ribbons having a width of 2 μm anddriven between an undeflected and a deflected position at frequency ofabout 10 kHz. Line 804 shows the full deflection or stroke of theribbons of about 700 nm, and line 806 shows the optical response(intensity of light reflected) of the ribbons when illuminated by lighthaving a wavelength of 2.8 nm. It is noted that the simulation utilizes2.8 μm incident light to enable the intensity of modulated light fromthe SLM to be extinguished at quarter wave deflection (2.8 μm/4 isapproximately 700 nm), thereby demonstrating the optical response of theribbon-type SLM with the electrically permeable damping structure.Referring to FIG. 8, it is noted that the optical pulses in line 806 aresubstantially at square with minimal ringing 808 demonstrating theefficacy of the electrically permeable damping structure in improvingsettling time, while maintaining a high switching speed andsubstantially preventing pull-in or snap-down.

In contrast, for purposes of comparison FIG. 9 illustrates a response ofa ribbon-type SLM similar to that of FIGS. 5 and 7, having the sameribbon width (2 μm), but without the damping structure formed by thedielectric layer 512 and first air-gap 716, and operated at the samefrequency (10 kHz) and with the same stroke as used in deriving theresponse shown in FIG. 8. Referring to FIG. 9, line 902 shows the drivevoltage applied to ribbon, line 904 shows the deflection of the ribbons,and line 906 shows the optical response. Referring to FIG. 9, it isnoted that that the ribbon oscillation is heavy, resulting insubstantial amounts of ringing 908, rendering the SLM unusable at drivefrequencies or frame rates of 10 kHz.

FIG. 10 illustrates a simulated improved dynamic response of theribbon-type SLM of FIG. 7 with the electrically permeable dampingstructure operated at a frequency or frame rate greater than 100 KHz.Referring to FIG. 10, line 1002 shows the drive voltage applied toribbons having a width of 2 μm and driven between an undeflected and adeflected position at frequency of about 100 kHz. Line 1004 shows thedeflection or stroke of the ribbons, and line 1006 shows the opticalresponse. Referring to FIG. 10, it is noted that the optical pulses inline 1006 are still substantially square, clearly superior to theresponse of the undamped ribbons shown in FIG. 9, even when driven at afrequency 10 times greater than the conventional SLM of FIG. 9, thusdemonstrating the efficacy of the electrically permeable dampingstructure in improving settling time, while maintaining a high switchingspeed and substantially preventing pull-in or snap-down.

In another aspect, an efficient method for driving the capacitive MEMSdevice is provided. Generally, the method involves ganging together alarge group of MEMS pixels, each including a plurality ofelectrostatically deflectable narrow ribbons, and driving each group ofMEMS pixels with a repetitive pattern to steer a beam of light reflectedfrom the ribbons. An embodiment of this method for driving a MEMS-basedphased-array will now be described with reference to FIGS. 11A and 11B,and FIG. 12.

FIGS. 11A and 11B illustrate how increasing the period of pixels in aribbon-type SLM used in phased-array beam scanning increases angularsteering. Referring to FIG. 11A, in order to steer a beam 1102, ribbons1104 are arranged in a “blaze” pattern 1106 of pitch or period A toreflect light incident on the ribbons at an normal angle of incidence atan angle θ. Generally, the blaze period is dependent on the number ofribbons in a MEMS pixel, where each pixel repeats the blaze pattern. Byregularly and continuously changing the pitch of the blaze pattern 1106,angular sweep of the reflected light is achieved.

The angle by which the light is reflected from the ribbon-type SLM in ablazed configuration is related to the period A and to a wavelength (λ)of the incident light is given by:

Sin θ=λ/Λ

Thus, it is noted that a steering angle θ, or angle by which incidentlight is reflected is strong inversely related to blaze pitch (Λ), whilelong period gratings add angular resolution where resolution is alreadygood (i.e. around mirror surface normal). FIG. 11B illustrates theangular swing for a various blaze periods for light at a wavelength (λ)of about 980 nm. Referring to FIG. 11B, line 1108 represents the angularswing that can be achieved by a ribbon-type SLM having a blaze period of8192 ribbons, each having a width of 2.5 nm. The portion of line 1108indicated by reference numeral 1110 represents the angular swing thatcan be achieved by a blaze period of 1024 ribbons. The portion of line1108 indicated by reference numeral 1112 represents the angular swingthat can be achieved by a blaze period of 512 ribbons. The portion ofline 1108 indicated by reference numeral 1114 represents the angularswing that can be achieved by a blaze period of 256 ribbons. It isobserved from FIG. 11B that most of angular swing is achieved with shortblaze periods. For example, 100 ribbons achieves 0.1° minimum angle.

The physical length of the array is also important in many phased-arrayapplications. It has been found that longer arrays, with a greaternumber of ribbons, provide better angular resolution. However, as notedabove, ribbon width is typically minimized in order to maximize angularrange. Thus, for a phased-array device it is desirable to provide aribbon-type SLM having a large number of ribbons, and with shorter blazeperiods.

Conventional and prior art ribbon-type SLM used in diffract imagingtypically include smaller arrays with a lower number of ribbons, and asa result typically assign a single CMOS driver or drive channel to eachto each ribbon. While this approach is conceptually straightforward, itis unnecessary and inefficient for ribbon-type SLM used for beamsteering in which the same highly periodic blaze pattern repeated manytimes along the array. Moreover, long period blaze patterns are notnecessary because they only increase resolution near normal(zero-degrees), providing more resolution where angular resolution isalready good. Thus, dedicated drivers for each ribbon are inefficient,resulting in excessive data rate, power consumption, size and cost.

A better approach is to gang larger groups of ribbons with blazedgrating to a much smaller number of electronic drivers. One embodimentof this approach is shown in FIG. 12. FIG. 12 is a schematic blockdiagram of a ribbon-type SLM illustrating a portion of the ribbon array1202, a number of drivers or drive channels 1204 and driver bus 1208.Referring to FIG. 12, it is seen that each of the drive channels 1204are interlaced and interconnected to multiple ribbons 1210 in the array1202, such that each drive channel is coupled through the driver bus1208 to only one ribbon 1210 in each one of multiple pixels 1212 in thearray. It is noted that in the embodiment shown in FIG. 12 forillustrative purpose the number of drive channels 1204 and ribbons 1210are limited to eight. However, it will be understood that in actualimplementation the number of drive channels 1204, pixels 1212, andribbons in the array 1202 can be any number desired for the applicationin which the ribbon-type SLM is to be used. For example, in oneembodiment particularly suitable for use in LIDAR, the array 1202 caninclude hundreds of pixels 1212, each including 136 ribbons 1210 driventhrough a driver bus with 136 conductors by 136 drive channels. In thisembodiment, the longest blaze period is equal to the number of channeldrivers. Shorter blaze periods can be achieved by programming driverswith repetitive patterns. This design economizes on data rate, powerconsumption, module size and cost, while preserving good light gatheringaperture and high angular resolution. The implementation of this designrequires good device uniformity (i.e. no individual pixel calibration).

Thus, embodiments of a spatial light modulator (SLM), including, andmethods for operating the same for efficient data transmission have beendescribed. Although the present disclosure has been described withreference to specific exemplary embodiments, it will be evident thatvarious modifications and changes may be made to these embodimentswithout departing from the broader spirit and scope of the disclosure.Accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of one or more embodiments of the technicaldisclosure. It is submitted with the understanding that it will not beused to interpret or limit the scope or meaning of the claims. Inaddition, in the foregoing Detailed Description, it can be seen thatvarious features are grouped together in a single embodiment for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimedembodiments require more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

Reference in the description to one embodiment or an embodiment meansthat a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe circuit or method. The appearances of the phrase one embodiment invarious places in the specification do not necessarily all refer to thesame embodiment.

What is claimed is:
 1. A method of fabricating a microelectro-mechanical systems (MEMS) device comprising: forming a bottomelectrode over a substrate; forming a first sacrificial layer over thebottom electrode; forming a dielectric layer over the first sacrificiallayer; forming a second sacrificial layer over the dielectric layer;forming a plurality of ribbons over the second sacrificial layer; andetching the first and second sacrificial layer with a gas to form afirst air-gap between the dielectric layer and the bottom electrode, anda second air-gap between the dielectric layer and the plurality ofribbons such that each of the plurality of ribbons are free to deflecttowards the bottom electrode upon application of a voltage potentialbetween the bottom electrode and the ribbon.
 2. The method of claim 1wherein forming the dielectric layer comprises forming the dielectriclayer to substantially surround and overlie the first air-gap uponetching the first sacrificial layer.
 3. The method of claim 1 whereinthe first sacrificial layer comprises a thickness to provide a firstair-gap having a thickness of 2G/3 separating the dielectric layer fromthe bottom electrode, where G is a distance between the bottom electrodeand the plurality of ribbons when undeflected.
 4. The method of claim 3wherein an undeflected movable member is separated from the dielectriclayer of the electrically permeable damping structure by a distance ofG/3.
 5. The method of claim 1 wherein the plurality of ribbons are freeto deflect towards the bottom electrode by a distance substantiallyequal to a thickness of the second air-gap separating the dielectriclayer from the plurality of ribbons.
 6. The method of claim 1 whereinthe plurality of ribbons are free to deflect towards the bottomelectrode by a distance of up to 2 μm.
 7. A method for driving a microelectro-mechanical systems (MEMS) modulator including an array of aplurality of MEMS pixels comprising: interconnecting the plurality ofMEMS pixels under control of a single, integrated programmable driverincluding a plurality of drive channels; interlacing the plurality ofdrive channels, such that each drive channel is coupled to one ribbon ineach one of the plurality of MEMS pixels; and operating the integratedprogrammable driver to monotonically vary deflection of each ribbon ineach of the plurality of MEMS pixels to produce a repetitive patternacross the array of the plurality of MEMS pixels to steer a beam oflight modulated by the MEMS modulator.
 8. A Light Detection and Ranging(LiDAR) system comprising: a microelectro-mechanical systems (MEMS)device including: a bottom electrode formed over a substrate; anelectrically permeable damping structure formed over the bottomelectrode, the electrically permeable damping structure including afirst air-gap and a dielectric layer suspended above and separated fromthe bottom electrode by the first air-gap; and a plurality of movablemembers suspended above the damping structure and separated therefrom bya second air-gap, each of the plurality of movable members including atop electrode and being configured to deflect towards the bottomelectrode by electrostatic force.
 9. The LiDAR system of claim 8 whereinthe plurality of movable members comprise a plurality of independentlydeflectable ribbons, each of ribbon comprising a reflective surface tomodulate light incident on the MEMS device.
 10. The LiDAR system ofclaim 9 wherein the plurality of independently deflectable ribbons areconfigured to deflect towards the bottom electrode by a distancesubstantially equal to a thickness of the second air-gap.
 11. The LiDARsystem of claim 9 wherein the thickness of the second air-gap is up to 2μm.
 12. The LiDAR system of claim 9 wherein the dielectric layersubstantially surrounds and overlies the first air-gap.
 13. The LiDARsystem of claim 9 wherein each of the plurality of independentlydeflectable ribbons comprise width of from 2.5 μm to 0.5 μm.
 14. TheLiDAR system of claim 8 wherein the dielectric layer is separated fromthe bottom electrode by a distance of 2G/3, where G is a distancebetween the bottom electrode and the plurality of movable members whenundeflected.
 15. The LiDAR system of claim 14 wherein an undeflectedmovable member is separated from the dielectric layer of theelectrically permeable damping structure by a distance of G/3.